Preionization arrangement for a gas laser

ABSTRACT

A preionization device for a gas laser comprises an internal preionization electrode having a dielectric housing around it and an external preionization electrode displaced from the dielectric housing by a small gap. The dielectric housing includes two cylindrical regions of differing outer radii of curvature. An open end of the housing has a larger radius of curvature than the other end which is closed. The internal electrode connects to circuitry external to the discharge chamber via a conductive feedthrough which penetrates through the housing. The external circuitry prevents voltage oscillations caused by residual energy stored as capacitance in the dielectric housing. The external preionization electrode, which is connected electrically to one of the main discharge electrodes, is formed to shield the internal preionization electrode from the other main discharge electrode to prevent arcing therebetween. The external electrode is also formed to shield the outer gas volume and walls of the discharge chamber from the preionization unit. A semi-transparent external electrode prevents charged particles emanating from the main discharge area from settling on the housing and causing field distortion and discharge instabilities.

PRIORITY

[0001] This application is a divisional application which claims thebenefit of priority to U.S. patent application Ser. No. 09/247,887,filed Feb. 10, 1999.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to an excimer laser pumped by an electricalgas discharge, and particularly to a preionization device and techniquefor generating a stable pulsed gas discharge for pumping of an activemedium of an excimer laser.

[0004] 2. Discussion of the Related Art

[0005] UV-preionization of the electrical discharge in a pulsed gaslaser is typically realized by means of an array of spark gaps or byanother source of UV-radiation (surface, barrier or corona gasdischarges), disposed in the vicinity of at least one of the solidelectrodes of the main discharge of the laser. Conventional pulsedelectrical gas discharges typically used for pumping the active media ofexcimer lasers are unstable. The development of discharge instabilitiescause the glow discharge, a precondition for laser emission, to have ashort phase (e.g., having a typical duration from 10-100 ns) and to thusbe terminated too quickly. The desired way of generating a high qualitygas discharge for use in excimer lasers is to provide an intense, yetuniform preionization of the gas volume before the main gas dischargeoccurs. One way of providing this preionization is by photo-ionizing thelaser gas with UV-light emitted from an auxiliary gas discharge beforethe main gas discharge is switched on. Some known methods of preionizinghigh pressure gas lasers include x-ray, spark and corona-gappreionization. See R. S. Taylor and K. E. Leopold, Pre-preionization ofa Long Optical Pulse Magnetic-Spiker Sustainer XeCl Laser, Rev. Sci.Instum. 65 (12), (December 1994). The spark method involves the use ofspark gaps (ordinary or stabilized by a dielectric surface), and thecorona-gap method involves the use of pulsed corona-like discharges neara dielectric surface.

[0006] Areas of focus for design improvement of corona-gap preionizersinclude the geometry of the dielectric body, and the arrangement of thepreionization electrodes. See U.S. Pat. No. 4,718,072 to Marchetti etal. (showing a grounded internal preionization electrode surrounded by adielectric having a positive potential applied to its outer surfacethrough contact with the positively biased main electrode); EuropeanPatent Application (published) EP 0 532 751 A1 (showing an internalpreionization electrode surrounded by a dielectric buried in one of themain electrodes); U.S. Pat. No. 4,953,174 to Eldridge et al. (showingthe dielectric surrounding an internal preionization electrode abuttingwith a main discharge electrode); see also R. Marchetti et al., A NewType of Corona-Discharge Preionization Source for Gas Lasers, J. Appl.Phys. 56 (11), (Dec. 1, 1984); U.S. Pat. No. 4,380,079 to Cohn et al.

[0007] Reconfiguration of external electrical circuits is another areawhere corona-gap pre-ionizer design improvement efforts have beenfocused. See Taylor et al., citation above; U.S. Pat. No. 5,247,531 toMuller-Horsche (showing an excitation of preionization electrodesaffected by the same high voltage source as the main dischargeelectrodes including a time delay inductance between them), U.S. Pat.No. 5,247,534 to Muller-Horsche (including flow bodies configured tofacilitate laser gas flow and formed of material exhibiting secondaryx-ray emission characteristics) and U.S. Pat. No. 5,247,535 toMuller-Horsche (disclosing electron emission from a heated cathode,wherein x-rays emitted as the electrons impinge upon a separate anodeserve to preionize the laser gas volume).

[0008] U.S. Pat. No. 5,337,330 to Larson describes the typicalcorona-like preionization arrangement of FIG. 1a. See also U.S. Pat. No.5,247,391 to Gormley, and U.S. Pat. No. 4,953,174 to Eldridge et al. Thedischarge chamber having the preionizing device of FIG. 1a includes ahigh voltage main electrode 1 and a grounded main electrode 2. Thepreionization unit includes two internal preionization electrodes 3 aeach located on one side of main discharge region 5 between the maindischarge electrodes 1,2. Each preionization unit includes a dielectrictube 3 b of generally cylindrical shape surrounding the internalpreionization electrode 3 a. A preionization discharge (ultravioletemission) 4 from the preionization electrodes 3 a & 6 and dielectrictubes 3 b causes a preionization of the volume of the main gasdischarge. A pair of external preionization electrodes 6 of thepreionization units comprise metal plates and are each directlyconnected to the nearby main discharge electrode 1 (e.g., the cathode athigh potential).

[0009] In this case energy stored in the dielectric tubes 3 b, which canbe non-negligible relative to the energy of the main discharge, during apreionization phase, will also be absorbed into the main discharge 5.However, that added energy typically will not increase the laser outputdue to a high wave impedance of the dielectric tubes 3 b. The tubes 3 bact much like a charged transmission line in that this wave impedance istypically much higher than the impedance of the main gas discharge. Thehigh wave impedance is caused by a distributed inductivity of the wholedielectric tubes 3 b (as a transmission line) and a concentratedinductivity at the point of electrical connection of the tubes 3 b withthe internal corona discharge electrodes 3 a.

[0010] The residual energy produces high voltage electrical oscillationsbetween the capacitance of the dielectric tubes 3 b of the preionizationunits and the main gas discharge volume. These high voltage oscillationsare undesirable because they significantly reduce the ability of thedielectric tubes 3 b of the preionization unit to resist direct highvoltage breakdown and over-flashing near the open ends of the dielectrictubes 3 b. Moreover, these oscillations deteriorate the quality of themain gas discharge 5 and thus hinder the operation of the laser,particularly during operation at a high repetition rate. Furthermore,the oscillations cause additional wear to the main gas dischargeelectrodes 1,2 and the internal corona discharge electrodes 3 a, andalso cause contamination and a reduced lifetime of the laser system.

[0011]FIG. 1b shows a conventional preionization unit setup wherein onlyone internal corona-discharge preionization electrode 3 a is employed.See U.S. Pat. No. 4,240,044 to Fahlen et al. FIG. 2 shows a perspectiveview of a preionization unit of either of FIGS. 1a and 1 b. Thepreionization unit includes the internal electrode 3 a and the externalelectrode 6. The area of most intense discharge 4 is shown at thesurface of the dielectric tube 3 b nearest the external electrode 6.

[0012] Another problem with conventional corona-like preionization unitsis illustrated in FIG. 3. In the preionization unit of FIG. 3, aninternal preionization electrode 3 a is shown surrounded by a dielectrictube 3 b. An external preionization electrode 6 is shown abutting thesurface of the dielectric tube 3 b. The dielectric tube 3 b oftenexhibits an unsatisfactorily non-uniform surface discharge 4 a in thisconfiguration. The non-uniform surface discharge leads to instabilitiessuch as arcing from areas of higher charge density. The lack ofuniformity of surface discharge also can cause an unstable “jitter” ofthe laser output. This jitter is a fluctuation of the interval betweensuccessive laser pulses from an evolving instability in the ignitionfrom one laser pulse to another. This variance, or jitter, isundesirable and makes laser performance less reproducible.

[0013] Other problems are associated with conventional corona-likepreionization units such as that illustrated in FIG. 4. Some of theUV-light emanating from the outer surface of the dielectric tube 3 bunit illuminates the main discharge volume 5, as is desired. However,some of the gas volume outside of the main discharge region 5 is alsoilluminated by the UV-light. The UV-light is preionizing a larger gasvolume than is either required or desired.

[0014] A disadvantage related to this is illustrated in FIG. 4, whichshows that at high repetition rate operation, arcing occurs across thegas volume between the external electrode of the preionizer 6 and thegrounded main electrode 2. Arcing of this kind puts constraints on themaximum achievable repetition rate. Moreover, even before the onset ofvisible arcing of this kind takes place, the laser pulse energy issubstantially reduced by parasitic discharges in the additionallypreionized gas volume. These parasitic discharges produce an instabilityin the laser operation.

[0015] Moreover, as may be understood from inspection of the arrowspointing away from the tube 3 b of FIG. 4, some UV-light is undesirablymisdirected away from the main discharge region 5 and is absorbed by thedielectric laser chamber walls. As a result, charges build up on thewalls and further inefficient arcing and parasitic discharging occurs.To address this problem, Japanese Patent Application No. 3-9582 and U.S.Pat. No. 5,337,330 to Larson each disclose a shielding element, shown asreference numerals 6 and 36, respectively, to reduce the electric fieldstrength between the main electrode and the dielectric pipe.

SUMMARY OF THE INVENTION

[0016] It is accordingly an object of the invention to design apreionization unit for a laser having a high quality gas discharge byproviding an intense, yet uniform, preionization of the gas volumebetween the main discharge electrodes.

[0017] It is also an object of the invention to provide a dielectrictube which prevents over-flashing and arcing at the tube ends.

[0018] It is another object of the invention to prevent electricaloscillations from arising out of residual energies stored in thedielectric tube.

[0019] It is an object of the invention to provide an external electrodewhich shields the walls of the discharge chamber and the gas volumeoutside of the main discharge area from the effects of the preionizationunit.

[0020] The present invention meets these objects and addresses theshortcomings of conventional preionization techniques by providing apreionization device for a gas laser which comprises an internalpreionization electrode having a dielectric tube around it and anexternal preionization electrode displaced from the dielectric housingby a small gap. The dielectric tube includes two cylindrical regions ofdiffering outer radii of curvature. One end of the tube is open to allowan electrical connection to the internal electrode, and the other end isclosed. The open end of the tube has a larger radius of curvature thanthe closed end. The internal electrode connects to circuitry external tothe discharge chamber at the open end of the tube via a conductivefeedthrough which penetrates through the housing. The external circuitryprevents voltage oscillations caused by residual energy stored ascapacitance in the dielectric housing. The external preionizationelectrode, which is connected electrically to one of the main dischargeelectrodes, is formed to shield the internal preionization electrodefrom the other main discharge electrode to prevent arcing therebetween.The external electrode is also formed to shield the outer gas volume andwalls of the discharge chamber from the preionization unit. Asemitransparent external electrode prevents electrical field distortionnear the main gas discharge.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1a shows a conventional arrangement of a discharge chamber ofa high repetition rate pulsed discharge laser having two internal andexternal preionization electrode pairs and a UV-preionized activevolume.

[0022]FIG. 1b shows the arrangement of FIG. 1a having instead only oneinternal and external electrode pair.

[0023]FIG. 2 shows an axial geometry of a conventional UV-preionizationunit having a gas discharge stabilized by a dielectric surface.

[0024]FIG. 3 shows a conventional UV-preionizer having an externalelectrode abutting the dielectric tube surrounding the internalelectrode.

[0025]FIG. 4 shows a UV-preionizer arrangement having conventionalexternal electrodes.

[0026]FIG. 5 shows a UV-preionizer in accord with the present inventionwherein a gap exists between the external electrode and the dielectrictube surrounding the internal electrode.

[0027]FIG. 6a shows a side view of a preferred embodiment of thedielectric tube surrounding the internal electrode of the UV-preionizerof the present invention.

[0028]FIG. 6b shows an end view of the tube of FIG. 6a looking towardthe open end of the tube.

[0029]FIG. 6c shows an end view of the tube of FIG. 6a looking towardthe closed end of the tube.

[0030]FIG. 6d shows a side view of an alternative embodiment wherein thedielectric tube is fed through an opening in the wall of the dischargechamber.

[0031]FIG. 7a shows a cross-sectional axial view of a discharge chamberof a gas discharge laser in accord with the present invention.

[0032]FIG. 7b shows a cross-sectional side view of the discharge chamberof FIG. 7a.

[0033]FIG. 8 shows a UV-preionizer arrangement configured with externalelectrodes in accord with the present invention.

[0034]FIG. 9 shows a preionization unit wherein a metal ring is providedat the end of the external electrode in accord with the presentinvention.

[0035]FIG. 10 shows a cross-sectional axial view of a preionizerarrangement including a semi-transparent external electrode in accordwith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Gap

[0036]FIG. 5 shows a UV-preionizer in accord with the present inventionwherein a gap 14 exists between the external electrode 17 and thedielectric tube 3 b surrounding the internal electrode 3 a. Thepreionizer configuration of FIG. 5 differs from the conventionalpreionizer configuration shown in FIG. 3 because that of FIG. 3 does notinclude the gap 14. The size of the gap is preferably in a range from 10to 200 microns.

[0037] The small additional gap 14 between the external electrode 17 ofthe preionizer and the outer surface of the dielectric tube 3 bsurrounding the internal electrode 3 a produces an area 15 of intensebarrier discharge, causing the surface of the dielectric tube 4 b to beilluminated with UV radiation substantially uniformly along its length.The gap 14 between the tube 3 b and the external electrode 17significantly improves laser performance and produces a more uniformpreionizing surface discharge 4 b than the surface discharge 4 aproduced using the conventional arrangement of FIG. 3. Weak plasmas ofthe barrier discharge area 15 fill the gap 14 during the initialformation phase of the preionization discharge and illuminate thesurface of the dielectric tubes, advantageously facilitating thedevelopment of a uniform surface discharge 4 b over the tube 3 b.

[0038] Also advantageously, this UV illumination reduces theabove-described jitter, further stabilizing the surface corona discharge4 b. That is, when the gap 14 is introduced in accord with the presentinvention, the fluctuation in time periods between pulses describedabove is significantly reduced. The preionization unit efficiency andoverall laser performance are enhanced because this pulse interval ismore uniform.

Dielectric Tube

[0039]FIGS. 6a-6 c show a configuration of a dielectric tube 3 bsurrounding an internal electrode 3 a of a UV-preionizer unit in accordwith the present invention. FIG. 6a shows a side view of the preferredtube 3 b. The dielectric tube 3 b comprises a material suitable for useinside a laser gas mixture which includes an aggressive halogen (e.g.,fluorine). The material of the tube 3 b is also capable of sustaining acapacitively coupled gas discharge. That is, the material has sufficientdielectric strength to support an intense surface electrical gasdischarge. For this reason, the preferred dielectric tube 3 b comprisesa ceramic such as Al₂O₃. The crystalline form (also known as sapphire),as well as the polycrystalline form, of Al₂O₃ may be used.

[0040] Dielectric tube preionizers must have reliable protection againstelectrical over-flashing from the external electrodes (not shown inFIGS. 6a-6 c) to the internal electrodes 3 a at each of the two ends16,18 of the dielectric tube 3 b. This over-flashing, or direct coronadischarging between the internal electrode 3 a and the externalelectrode 17, is undesirable because such discharging can terminate thepreionizing corona discharge along the dielectric tube 3 b, and triggersevere arcing at the tube ends 16,18 resulting in possible damage ordestruction of the dielectric tube 3 b due to increased energydissipation.

[0041] For at least this reason, the present invention provides anadvantageous dielectric tube 3 b having a closed end 16 and an open end18. FIG. 6b shows an end view of the dielectric tube 3 b looking towardthe open end 18 of the tube 3 b. FIG. 6c shows an end view of the tube 3b looking toward the closed end 16 of the tube 3 b.

[0042] The closure of the dielectric tube 3 b serves to electricallyinsulate the internal preionization electrode 3 a at the closed end 16and prevent direct flashing-over at that end 16. The end 16 ispreferably closed with the same dielectric material as the length oftube 3 b comprises. In this way, there is no undesirable change indielectric strength at the closed end 16 which might otherwise be causedby a change in dielectric constant. Changes in dielectric constant andstrength along the tube 3 b are advantageously avoided in the presentinvention to enhance uniformity and preionization efficiency.

[0043] The tube 3 b of the present invention, being comprised of ceramicand having a closed end 16, is advantageous for the additional reasonthat it is easily manufacturable. A tube 3 b comprising crystallineAl₂O₃, or sapphire, e.g., and open at both ends is preferably grown in afirst step, and then one end is closed in a second step. The crystalorientation in the entire tube 3 b including the closed end 16 may becontrolled to provide a tube 3 b having excellent dielectric homogeneityand strength.

[0044] A tube 3 b having a closed end 16 in accord with the presentinvention may have a shorter length than conventional dielectricpreionizer tubes without enhanced arcing from the internal electrode 3 aat the closed end 16 because the internal electrode 3 a is fullyinsulated at the closed end 16 by the dielectric tube 3 b. This lengthreduction is advantageous because the length of the preionizer tube 3 bis a constraint on the minimum length of the laser tube (not shown)itself. For example, the ratio of the length 1 a of the laser activegain volume to the total length it of the laser tube becomes closer tounity as the length of the preionizer tube 3 b is coincidentallyincreased. Generally, the larger the ratio l_(a)/l_(t) becomes, the moreefficient the laser system itself becomes. This is especially the casefor frequency narrowed high repetition rate lasers. For example:

[0045] l_(a)/l_(t)≈0.55 if preionizers with open ends are used; and

[0046] l_(a)/l_(t)≈0.75 if preionizers with one closed and one open endare used.

[0047] Only one end 16 of the tube 3 b can be closed because theinternal electrode 3 a must be connected to a defined external potentialor a pulse generator. The open end 18 thus has the opening 19 as shownin FIGS. 6a-6 c. Each of the surface corona intensity and flashing-overprobability depends significantly on the specific capacity (orcapacitance) of the dielectric tube 3 b. In turn, the specific capacityis approximately inversely proportional to the logarithm of the ratio ofexternal-to-internal dielectric tube diameters. Thus, an increase in theexternal-to-internal dielectric tube diameter ratio generally causes areduction of both the surface corona intensity and the flashing-overprobability. Therefore, as shown in FIG. 6a, the present inventionprovides a larger external diameter of the dielectric tube 3 b at theopen end 18 to provide a smaller capacitance at the open end 18. Theoptimum external diameter and length of the tube 3 b at the open end 18depends on several factors including the overall geometric design of thelaser, applied voltages, timing of voltage pulses, and the laser gasmixture. Some exemplary data include the following:

[0048] internal diameter of dielectric tube 3 b: 1 to 6 mm

[0049] external diameter of tube 3 b at closed end 16: 6 to 10 mm

[0050] external diameter of tube 3 b at open end 18: 20 to 30 mm

[0051] length of open end having larger external diameter: 50 to 75 mm

[0052] total length of tube 3 b: 0.5 to 1.0 m.

[0053] The external diameter of the tube 3 b at the open end 18 isconstrained by the proximity of the anodic main electrode (not shown inFIGS. 6a-6 c), and the probability of cascade electrical breakdown fromthe tube 3 b to ground. A tube 3 b having two length portions ofdiffering external diameters and homogeneous dielectric properties, suchas the preferred embodiment of the present invention, may bemanufactured using a two step growing process. The first step forms thefirst length with the first external diameter and the second step formsthe second length with the second external diameter. A third step mayinclude forming the closed end 16.

[0054] Instead of having a “thick” open end 18, the dielectric tube 3 bmay alternatively be used as a feedthrough. That is, the tube 3 b may besealably fed through an opening 21 in the dielectric wall 22 of thelaser discharge chamber, as shown in FIG. 6d. In this alternativeembodiment, the internal electrode 3 a within the feedthrough tube 3 bof the preionizer unit is easily connectable to an external potential orpulse generator. That portion 23 of the tube 3 b where a surface coronadischarge might start is now located external to the laser chamber.Advantageously, the surface corona intensity is strongly reduced in thiscase because the onset for a corona discharge is much lower in aircompared to the laser gas mixture. Moreover, the length of the tube 3 brequired for reliable prevention of overflashing is about 3 to 5 timesshorter in air than in the laser gas mixture. The total length of thelaser tube may thus be reduced.

[0055] Returning to the preferred embodiment shown in FIGS. 6a-6 c, theinternal electrode 3 a preferably connects with an external electricalpotential (not shown) at a connection point 20 at the very end of theinternal electrode 3 a which is shown protruding from the open end 18 ofthe tube 3 b. The enhanced thickness of the wall of the tube 3 b at theopen end 18 allows an external electrical potential to be applied to theinternal electrode 3 a of the preionizer at the connection point 20 witha reduced risk of “flashing over” from the internal electrode 3 a.

[0056] The internal electrode 3 a of the preionizer preferablysubstantially fills the entire internal space of the dielectric tube 3 bduring operation to facilitate strong capacitive coupling between thepreionization discharge and the internal electrode 3 a. This is becauseany space between the internal electrode 3 a and the dielectric tube 3 bshould be kept to a minimum. Only a small spacing should be provided toallow for any enhanced thermal expansion rate that the internalelectrode 3 a may exhibit over the dielectric tube 3 b during operation.

[0057] The internal electrode preferably comprises a metal tube or asolid metal rod. A thin walled flexible metal tube having a smalllongitudinal slit along the length of the tube may also be used. Thislatter “slit” design provides the above-described small spacing betweenthe internal electrode 3 a and dielectric tube 3 b wherein a thermallyinduced mechanical stress to the dielectric tube is minimized. The slitprovides the expansion space for the metal internal electrode 3 a. Theinternal electrode may also comprise a conductive liquid.

Feedthroughs

[0058]FIGS. 7A and 7B show an axial cross-sectional view and a sidecross-sectional view, respectively, of a laser discharge chamber inaccord with the present invention. The main discharge electrodes 1,2 aremutually opposed along the top and bottom of the discharge chamber,respectively. Two preionizer units are shown having internal electrodes3 a and external electrodes 3 b. The main discharge area 5 is betweenthe main electrodes 1,2. A pair of external electrodes 27 are shownconnected to the top main electrode 1, which is preferably the cathode,or high potential, main electrode 1. Feedthroughs 24 sealably penetratethe discharge chamber from the outside, and advantageously allowexternal circuitry 26 (external to the discharge chamber) to beconnected to the internal electrodes 3 a of the preionizer units. A setof peaking capacitors 25 is also shown. The peaking capacitors are usedfor electrical pumping of the main discharge. Their values depend on thedischarge and the configuration of the gas mixture and is usually in arange from around 5 nF to around 50 nF.

[0059] The internal electrodes 3 a of the preionizer unit areconventionally connected directly to the second main discharge electrode2, which is the anode at ground potential. This conventional approach,however, allows an undamped electrical oscillation to arise out ofresidual energy stored in the dielectric tube 3 b. For this reason, eachinternal electrode 3 a of the preionizer unit of the preferredembodiment of the present invention is connected to auxiliary circuitry26 located external to the discharge chamber via the feedthrough 24.This auxiliary circuitry 26 is preferably a resistor connected to groundpotential. The resistor of the auxiliary circuitry 26 has a resistancevalue comparable to or larger than a wave impedance of an oscillatingcontour of the preionizer. A typical wave impedance value for apreionizer unit is R_(w)=(L/C)^(0.5)=8 to 15 Ω. A preferred resistancevalue of the resistor is then R=30 to 70 Ω. Although the resistors ofthe external circuitry 26 play no significant role during thepreionization phase, they do serve to significantly damp theoscillations of the preionizer after the preionization phase. Thereduction or prevention of these undesirable voltage oscillationsenhances the reliability and increases the lifetime of the preionizerunit.

[0060] The external electrical circuitry 26 may include more complexpassive and/or active electrical components. This external circuitry 26connected to the internal electrodes 3 a may provide electrical pulsesof desired shape and periodicity. It is the feedthroughs 24 whichprovide the connection of the internal electrodes 3 a to these or any ofa wide variety of other useful external electrical devices 26.

[0061]FIG. 7B shows how the preionizers are fixed into position withinthe gas discharge chamber according to a preferred embodiment of thepresent invention. Several holders 28 spaced along the length of thepreionizer tube 3 b suspend the tube 3 b within the discharge chamber.The number of these holders 28 depends on the size and composition ofthe preionizer unit.

[0062] At least one holder 28 fastens each end of the tube 3 b andadditional intermediate holders 28 may be used. Use of these holdersprovides design flexibility such that the small gap 14 of preferably10-200 μm shown in FIG. 5 between the external electrodes 27 of thepreionization units and the dielectric tubes 3 b. The holders 28 of thedielectric tubes are preferably made of a dielectric material (forexample ceramics) to avoid disturbances of the electrical field by theholders 28.

External Electrode Configuration

[0063] Preionization involves a gas discharge near the surface of thedielectric tubes 3 b. This gas discharge is supported by capacitivecoupling between the internal electrodes 3 a and the external electrodes27. The internal electrodes 3 a and the external electrodes 27 are sonamed because of their positions inside and outside of the dielectrictube 3 b, respectively.

[0064] A typical problem which arises during the operation of pulsed gasdischarge lasers at high repetition rates is arcing 4, as illustrated inFIG. 4. Referring to FIG. 8, the performance of a gas discharge laser inaccord with the preferred embodiment of the present invention isimproved at high repetition rates because its external electrodes 27 areespecially designed to prevent or reduce this arcing 4.

[0065] The external electrodes 27 shown in FIG. 8 are connected to themain discharge electrode 1 and are made from a conducting material suchas thin sheet metal or metal foil preferably having a thickness in therange from 50 to 500 μm. From the main discharge electrode 1, eachexternal electrode 27 is formed to approach the tube 3 b on a path overthe top and around the outside of its associated tube 3 b containing theinternal electrode 3 a. Preferably, no portion of the externalpreionization electrode 27 lies between the dielectric tube 3 b and thefirst main discharge electrode 1. The external electrode 27 ispreferably shaped such that it surrounds the dielectric tube 3 b andopens to the main discharge area 5 between the first main dischargeelectrode 1 and the second main discharge electrode 2. The externalelectrode 27 preferably is interposed between the dielectric tube 3 band the adjacent chamber wall(s) the isolate the adjacent chamberwall(s).

[0066] Preferably, the external electrode has a portion 32 which runshorizontally away from the main electrode 1 above the tube 3 b. Anotherportion 34 beneath the tube 3 b and opposite the first portion 32 has anend 36 which is bent toward the tube 3 b. This bent end 36 is preferablyspaced from the tube 3 b by a constant gap along the length of the tube3 b.

[0067] The external electrodes 27 of the preionizer of the presentinvention are designed in the above way with many advantages. Theexternal electrodes 27 shield the tube 3 b from the second maindischarge electrode 2. This shielding reduces or prevents the arcing 4shown in FIG. 4 and results in superior performance. The arcing isreduced or prevented because the gas volume outside of the maindischarge area 5, and particularly near the walls of the gas dischargechamber where downstream arcing 4 usually develops, is not preionizeddue to the shielding effect of the external electrode 27.

[0068] Advantageously, the design of the external electrodes 27 of thepresent invention provides an enhanced illumination of the main gasdischarge volume because it is not undesirably screened by the externalelectrodes 27 as in the prior art (see FIG. 4). The design of theexternal electrode 27 of the present invention also makes it possible toachieve higher a repetition rate during operation without having toincrease gas flow velocity between the main electrodes 1,2.

[0069] The external electrodes 27 are generally shorter than theircorresponding dielectric tubes 3 b. The ends of conventional externalelectrodes 6 also typically are quite sharp, having radii of curvaturefrom 25 to 250 μm. This results in an enhancement of local electricalfield strength around the ends of conventional external electrodes.Consequently, the probability of arcing occurring between a conventionalinternal electrode 3 a and a conventional external electrode 6 isundesirably high. Enhancement of the local electrical field alsoincreases electrical stress of the dielectric tube 3 b and reduces itsreliability.

[0070] This field strength enhancement is avoided in the preferredembodiment of the present invention because the radius r of the ends ofthe external electrodes 27 is increased. A preferred way of achievingthis enhanced radius is by providing a metal ring 38 at the ends of theexternal electrodes 27, as illustrated in FIG. 9. The inner diametersd_(l) of the rings 38 preferably approximately equal the outsidediameter of the dielectric tube 3 b. The outside diameter d_(a) of therings 38 is thus significantly larger than the diameter of thedielectric tube 3 b. The radius of curvature r of the outer surface ofthe rings 38 is computed as r=(d_(a)−d_(i))/4. Typical values are:

[0071] d_(i)=6 to 10 mm;

[0072] d_(a)=10 to 15 mm;

[0073] r=1 to 2 mm,

[0074] and the radius of curvature r is sufficient to prevent arcingfrom the ends of the external electrodes 27.

Semi-transparent External Electrode

[0075] The electric field between the main discharge electrodes 1,2 of aconventional corona type preionizer arrangement is distorted by largeintrinsic gradients caused by the electrical field of the preionizerunit. The surface discharge over the tube 3 b causes this distortion ofthe electrical field in the area of the main discharge. That distortionis reduced by the modified nature of the external electrodes 27 of thepresent invention shown in FIG. 10 due to a semitransparent electrode 47which better shields the preionizer field from the main discharge area 5and grounded main electrode 2. That reduction in distortion results inmore stable laser operation, especially at high repetition rates.

[0076] The semi-transparent electrode 47 preferably at least partiallycomprises a wire mesh or a perforated foil. The semi-transparentelectrode 47 partially covers the preionizer unit, shielding the maindischarge area 5 from fields of the preionizer unit and preventingelectric field distortion there. The preferred arrangement shown in FIG.10 includes support elements 48 and 49. Support element 48 supports theexternal electrode and preferably comprises a metal or other conductor.Support element 49 supports the dielectric tube 3 b and preferablycomprises a ceramic or other insulator with appropriate dielectricproperties.

[0077] The objects of the invention set forth above are thus met. Thegap 14 between the external electrode 27 and the dielectric tube 3 b(see FIG. 5) provides a high quality gas discharge by providing anintense, yet uniform, preionization of the gas volume between the maindischarge electrodes. The gap 14 allows the surface 4 b of thedielectric tube 3 b to be uniformly illuminated with UV-radiation alongits length. The design of the dielectric tube 3 b having an open end 18and a closed end 16, wherein the open end 18 has an increased externalradius reduces over-flashing and arcing at each of the tube ends 16,18.Electrical oscillations arising out of residual energies stored in thedielectric tube are avoided by connecting the internal electrodes toexternal circuitry via the feedthroughs 24. The design of the externalelectrodes 27 shield the walls of the discharge chamber and the gasvolume outside of the main discharge area 5 from the effects of thepreionization unit, and provides an enhanced illumination of the maindischarge area. The semi-transparent electrode 47 reduces distortion ofthe electrical field in the area of the main discharge 5.

[0078] Those skilled in the art will appreciate that the just-disclosedpreferred embodiments are subject to numerous adaptations andmodifications without departing from the scope and spirit of theinvention. Therefore, it is to be understood that, within the scope ofthe appended claims, the invention may be practiced other than asspecifically described above.

What is claimed is:
 1. A discharge chamber for a gas discharge laser,comprising: a housing configured to hold a laser gas mixture; a pair ofopposed main discharge electrodes within said housing; a corona-typepreionization electrode within said housing; and an electric circuitexternal to said housing and electrically connected to saidpreionization electrode via a conductive feedthrough which penetratesthrough said housing.
 2. The discharge chamber of claim 1 , wherein saidelectric circuit includes a passive electrical component for dampingelectrical oscillations within said preionization electrode.
 3. Thedischarge chamber of claim 2 , wherein said passive electrical componentincludes a resistor.
 4. The discharge chamber of claim 3 , wherein saidresistor is connected to ground.
 5. The discharge chamber of claim 3 ,wherein said resistor has a resistance which is greater than orsubstantially equal to the wave impedance of the preionizationelectrode.
 6. The discharge chamber of claim 3 , wherein said resistorhas a resistance which is between 8 ohms and 70 ohms.
 7. The dischargechamber of claim 3 , wherein said resistor has a resistance which isbetween 30 ohms and 70 ohms.
 8. The discharge chamber of claim 1 ,wherein said electric circuit includes an active electrical componentfor providing an appropriate waveform of the voltage across thepreionization electrode.
 9. The discharge chamber of claim 8 , whereinsaid active electrical component includes a pulse generator.
 10. Thedischarge chamber of claim 1 , wherein the gas discharge laser is one ofan excimer laser and a F2-laser.
 11. A discharge chamber for a gasdischarge laser, comprising: a housing configured to hold a laser gasmixture; a pair of opposed main discharge electrodes within saidhousing; a corona-type preionization electrode within said housing; andat least one passive electrical component electrically connected to saidpreionization electrode for damping electrical oscillations within saidpreionization electrode.
 12. The discharge chamber of claim 11 , whereinsaid passive electrical component includes a resistor.
 13. The dischargechamber of claim 12 , wherein said resistor is connected to ground. 14.The discharge chamber of claim 12 , wherein said resistor has aresistance which is greater than or substantially equal to the waveimpedance of the preionization electrode.
 15. The discharge chamber ofclaim 12 , wherein said resistor has a resistance which is between 8ohms and 70 ohms.
 16. The discharge chamber of claim 12 , wherein saidresistor has a resistance which is between 30 ohms and 70 ohms.
 17. Adischarge chamber for a gas discharge laser, comprising: a housingconfigured to hold a laser gas mixture; a pair of opposed main dischargeelectrodes within said housing; a corona-type preionization electrodewithin said housing; and a resistance having a value that is greaterthan or equal to the wave impedance of the preionization electrode andelectrically connected to said preionization electrode for dampingelectrical oscillations within said internal electrode.
 18. Thedischarge chamber of claim 17 , wherein said resistor is connected toground.
 19. The discharge chamber of claim 17 , wherein the resistancehas a value which is between 8 ohms and 70 ohms.
 20. The dischargechamber of claim 12 , wherein the resistance has a value which isbetween 30 ohms and 70 ohms.